1. Related Applications
The present application is based on, and claims priority from, French Application Number 0504145, filed Apr. 25, 2005, the disclosure of which is hereby incorporated by reference herein in its entirety.
2. Field of the Invention
This invention relates to the parametric generation of a monochromatic wave from the interaction of two incident collinear monochromatic waves injected into a medium with non-linear optical properties.
One of the application fields of the invention is the band III of the spectrum; that is, the wavelengths included between 8 μm and 12 μm, for which a few coherent sources that can be matched are currently composed of semiconductor lasers such as quantum cascade lasers.
Nevertheless, the optical parametric generation offers a promising research area, even if it requires materials that are in shortage.
3. Description of the Prior Art
Thus, the electronic industry's usual semiconductors, such as the gallium arsenide GaAs, zinc selenide ZnSe or indium phosphide InP, are excellent candidates for the parametric conversion of an optical radiation of the band I of the spectrum; that is, wavelengths included between 1 μm and 3 μm, to band III. Indeed, the previous-mentioned semiconductors
(i) present non-linear efficiency levels from among the largest ones in optical materials,
(ii) have transparency spectral areas, and therefore spectral matching, that are very extensive, for example from 0.5 μm to 20 μm for ZnSe,
(iii) benefit from a very advanced technology, inherited from microelectronic technology,
(iv) are easily available in the world market and are not subject to any embargos, and
(v) are potentially inexpensive.
These materials are nevertheless isotropic, which prohibits any phase matching scenario by natural birefringence. To resolve this limitation, the article by J. A. Armstrong et al., “Interactions between Light Waves in a Nonlinear Dielectric”, Physical Review, Vol. 127, No. 6, p. 1918 to 1939, Sep. 15, 1962, proposes an alternative technique called from the quasi-phase matching (QPM) that perform an efficient conversion. The QPM consists of compensating the wave propagation phase-shift due to the index dispersion, by artificially and periodically adding an additional phase-shift. This periodic rephasing of waves is imposed after a periodic distance that defines segments along the material by the waves. These segments individually maximise the conversion efficiency and have a length equal to an odd number of coherence lengths.
According to the article by R. Haidar et al, “Fresnel phase matching for three-wave mixing in isotropic semiconductors”, Journal Optical Society of America, Vol. 21, No. 8, p. 1522 to 1534, August 2004, the quasi-phase matching QPM resorts to the total internal reflection in the nonlinear medium: the phase-shift that each wave undergoes upon reflection compensates the phase-shift due to the propagation in a nonlinear optical material. Phase matching is obtained by what is called the “Fresnel birefringence”, because each wave undergoes a phase-shift that depends particularly on its frequency and on its polarization. The distance between two “bounces” against the surfaces of a plate of nonlinear optical material is, each time, chosen strictly equal to an odd number of coherence lengths for a resonant QPM, or close to an odd number of coherence lengths for a non-resonant QPM in order to maximise the conversion on each basic segment between two successive bounces. The wave generated on output is therefore the result of the constructive interference between the individual contributions of all the waves.
The conversion efficiency η of a wavelength converter in QPM is proportional to the square of the number N of rephasing and therefore of the number of bounces in the case of the QPM by total internal reflection: η∝N2, regardless of the thickness of the material crossed. The conversion efficiency of the QPM nonetheless remains limited. In the best cases, for a given material length, the conversion efficiency cannot exceed 40% of the efficiency that would allow perfect phase matching PPM, obtained for example by natural birefringence.